Sex Education
I think it's something we have to talk about and reinforce from time to time. How boys and girls are different, why boys cannot anyhow touch girls even though it means playing rough and punching the girl in the chest as an accident during rough play, etc etc. I doubt it's going to be a once off thing where we talk about it and expect them to understand immediately and to remember for life.
I remember my mom talking to me about menses when I was like 9 years, I was totally clueless and keep expecting to see red urine or something... I got excited when I saw really orange pee (pardon me)...but I didn't get to have my period till when I was 12yrs... but from time to time, when I'm alone with my mom in the car, she would talk about not letting boys/men anyhow touch me, should learn to love myself...all the way till secondary school, she would still talk about it at times with me. But about sex, not much details on her part.
But more imptly she was promoting abstinence since I'm from a Christian background.
I think it's something I would highly encourage my boy too when he's that young, impressionable, puppy love age. I think all the more someone should be around at home, setting house rules like friends especially the opposite sex permitted in the living room and not bedroom, etc etc.
Like I mentioned, it's dependent on how "open" or "narrow-minded" we are on this matter.